The present invention relates to a vehicle dynamic control system of the type that employs an electrically assisted steering system (EAS) and has means to counter the effects of vehicle oversteer.
EAS systems are well known in the art. Electric assist steering systems that use, for example, a rack and pinion gear set to couple the steering column to the steered axle provided power assist by using an electric motor to either apply rotary force to a steering shaft connected to a pinion gear, or apply linear force to a steering member having rack teeth thereon. The electric motor in such systems is typically controlled in response to (a) driver's applied torque to the steering wheel, and (b) sensed vehicle speed.
Other known electric assist steering systems include electro-hydraulic systems in which the power assist is provided by hydraulic means under at least partial control of an electronic control system.
Oversteer is the tendency of a vehicle to steer into a sharper turn than the driver intends and where there can be a thrusting of the rear of the vehicle outwardly of the bend, causing the rear of the car to start to slide due to the rear tyres losing lateral traction. This type of event is considered the most dangerous condition for a vehicle to be in and frequently results in the vehicle spinning out of control. Vehicle Stability Control (VSC) systems have been developed that prevent this by selectively applying the brakes at individual wheels. Such systems are expensive and therefore only suited for fitment to higher end cars, whereas power steering systems are now fitted to almost all types of vehicle. Therefore an overseer assistance system that could use the steering system would not only assist VSC in stabilising the vehicle, but would also be of more widespread application.